The present invention relates to a method and apparatus for securing a turbine blade to a turbine rotor.
Modern steam and gas turbines generally employ blade and rotor designs which provide for mechanical attachment or securing of a turbine blade to a turbine rotor. Generally, several turbine blades are attached to a singler rotor.
Conventional attachment schemes for attaching one of the blades to the rotor typically include an elongated triangular blade root which extends from the base of the blade, and a mating elongated triangular cavity provided in the rotor (the blade root and the cavity are triangular in cross-section). The outer periphery of the blade root is typically provided with a wavy configuration forming a plurality of outwardly extending lugs and inwardly directed grooves. Similarly, the wall of the rotor cavity is provided with a wavy configuration forming a plurality of outwardly extending lugs and inwardly extending grooves. When attached, the blade root is fitted into the rotor cavity such that the outwardly extending lugs of the blade root extend into the inwardly extending grooves of the cavity wall, and the outwardly extending lugs of the cavity wall extend into the inwardly extending grooves of the blade root.
FIG. 1 shows a cross section of a portion of a conventional turbine rotor 10 and several conventional blades 12 attachable to turbine rotor 10. As shown in FIG. 1, each blade 12 includes a base portion 14 from which a blade root 16 extends. As described above, turbine rotor 10 is provided with cavities 18 in which blade roots 16 extend.
Rotor 10 has an outer peripheral surface 20 in which several blade root grooves or cavities 18 are provided. The portions 22 of rotor 10 which are located between cavities 18 are typically called disc steeples.
Also as described above, the outer peripheral surface of each blade root 16 is provided with a wavy configuration forming several outwardly extending lugs 24 and several inwardly extending grooves 26. The walls of each cavity 18 are also provided with a wavy configuration forming several outwardly extending lugs 28 and inwardly extending grooves 30. When attached, the outwardly extending lugs 24 of blade root 16 extend into inwardly extending grooves 30 of the walls of cavity 18. Also, outwardly extending lugs 28 of the walls of cavity 18 extend into inwardly extending grooves 26 of blade root 16. Optimally, blade root 16 is fitted tightly or snugly within cavity 18, such that no, or a minimum amount of, clearance exists between blade root 16 and disc steeples 22. Such a tight or snug fit insures that blade 12 will not move or vibrate with respect to rotor 10 during operation of the turbine. Additionally, such tight or snug fitting insures that blade 12 maintains a proper alignment (e.g., radial alignment) with respect to rotor 10 and/or with respect to the cavity 18.
Referring again to FIG. 1, each groove 30 is provided with a first surface 32 (located at the upper portion of each groove 30 shown in FIG. 1) and a second surface 34 (located at the bottom portion of each groove 30 shown in FIG. 1). First surfaces 32 extend into the wall of cavity 18 at an angle .alpha. with respect to the central axis of cavity 18. Second surfaces 34 extend into the wall of cavity 18 at an angle .beta. with respect to the central axis of cavity 18. Preferably, the angle .alpha. is greater than the angle .beta..
Similarly, outwardly extending lugs 24 of each blade root 16 include first surfaces 36 (located on the upper portion of each lug 24 shown in FIG. 1) and second surfaces 38 (located at the lower portion of each lug 24 shown in FIG. 1). First surfaces 36 extend at an angle .alpha. with respect to the central axis of blade root 16 and second surfaces 38 extend at an angle .beta. with respect to the central axis of blade root 16. This arrangement is intended to provide sufficient contact area between the surface 36 of each lug 24 and surface 32 of each groove 30 of each cavity 18. In this manner, operating stresses are exerted primarily between surface 32 of each groove 30 and surface 36 of each lug 24.
Although this design has been successful for a number of years, such problems as cracking of the blade root lugs tend to occur. Such cracking problems have been attributed to improper seating of lugs 24 within grooves 30. This problem has been found to be exacerbated by various operations carried out during turbine overhauls.
Such turbine overhaul operations tend to cause the groove and lug profile of cavities 18 to lose dimensional tolerances. That is, such turbine overhauls tend to change the shape or dimension of grooves 30 and lugs 28 provided in the walls of cavities 18 by removing portions of, or wearing away, the metal forming disc steeples 22. As a result, a blade root 16 inserted in cavity 18 of an overhauled turbine rotor 10 may not fit snugly within cavity 18.
Such loose fitting of blade root 16 within cavity 18 may allow blade 12 to vibrate or move with respect to rotor 10 during the operation of the turbine. This movement or vibration of a rotor blade 12 with respect to a rotor 10 can cause excessive damage to the walls of cavity 18 and to blade root 16 Also, such movement or vibrations can cause excessive frictional heating between blades 12 and rotor 10 and/or with respect to the cavity 18.
Prior methods of alleviating problems associated with a loosely fitting blade have included the use of a conventional metal shim placed between the lowermost portion (with respect to FIG. 1) of blade root 16 and rotor 10. However, since a rotor overhaul creates a loss of metal which is usually non-uniform about grooves 30 and lugs 28, current shimming techniques often result in blades 12 not being radially aligned or centered in cavity 18 and in blades 12 not seating tightly in cavities 18.